Variable-rate PGR shows promise in Texas cotton

Elton Robinson Farm Press Editorial Staff | Mar 04, 2005

There could be a benefit in the variable-rate application of plant growth regulators in cotton fields which show significant variations in growth, according to Stephan Maas, professor of Agricultural Microclimatology, Texas Tech University. Maas was a speaker at InfoAg Mid-South, a regional precision agriculture conference held in Tunica, Miss.

Maas explained that in an ideal situation, if there is little variability in plant maturity in a field, blanket applications of plant growth regulator can very effectively “shift the growth of the cotton plant from vegetative to reproductive, to make more bolls.

“Mepiquat chloride can be applied at low rates beginning around pinhead square, which is the mark of the beginning of the reproductive growth stage in cotton. Later on, it can be applied in higher rates when you get into bloom.”

Maas stressed that yield response depends on the condition of cotton plants at the time of application.

“If mepiquat chloride was applied to plants that showed a vegetative tendency during the early part of the reproductive stage, the studies indicated a good response. But there was a negative yield response when it was applied to plants not showing a vegetative tendency.

“Obviously, you don't want to treat plants with a plant growth regulator if they don't need it. That's when you're going to run the risk of having a potential decrease in yield.”

Maas noted that the southern High Plains and Texas Panhandle are gently rolling areas where there is quite a bit of cotton plant variability across the fields “and the yields we get across the fields.”

“It's also very likely that vegetative tendency is going to vary across the field. So we may need to use different plant growth regulator rates in different parts of the field. In fact, some parts of a field may have plants that should not have plant growth regulator applied at all.”

To measure variability across the field and not spend a lot of money, Maas worked with a farmer and a consultant who were using remote sensing technology to create multi-spectral images of the grower's cotton fields.

The images, which were taken in July, were further manipulated to indicate plant biomass, or an enhanced difference vegetation index. “The effect is that you can see all sorts of details in the fields that you wouldn't normally see.”

That image can be further manipulated to show different zones of vigor in the crop, according to Maas. “Mainly, the image shows that higher vigor zones have denser canopies. The plants are wider, and they cover more of the soil surface. These areas of growth correlate to taller plants unless you have different planting densities across the field.”

Plant height is an indicator of when to apply plant growth regulator. “So we can identify areas that have similar plant growth in terms of potential for vegetative tendency.”

Areas or management zones that look like they have the same biomass were manually drawn onto the computer when the project was first undertaken, according to Maas. “Now we let the computer draw those lines. It does a better job and makes fewer mistakes than we do.

“The next step is to take the image that we drew those zones on and load it into a hand-held iPac with GPS that displays the image. That allows us to walk into each of these zones in the field and make a determination on plant growth regulator applications, based on plant height and the height-to-node ratio.”

After the farmer has determined the correct plant growth regulator rates for the various zones, he takes the information back to the farm office, puts the information into the GIS for creating an application map showing the various rates recommended.

A card is put into a controller for the spray rig and the variable-rate application is made. The equipment is also logging the rate being applied, which can be used to create a map showing the application, known as an as-applied map.

“One of the first things we found when we did the as-applied map was there was a lag time when the driver crossed management zones. It wasn't able to respond as quickly as we had hoped. Right now, the farmer is trying to rectify that by making changes in the application equipment.”

Maas said there were significant cost savings in the variable-rate applications due to an overall reduction in the amount of plant growth regulator applied.

In addition, “there are potential cost savings if you don't have to drive the equipment in parts of the field that don't need treatment. If you have a big part of the field that doesn't need anything, why drive the tractor through it?”

When more data is collected, “we may reach the point where we will be able to go directly from enhanced imagery in the GIS to the PGR application rate and not have to do as much field scouting. We may do that as a check to our technique.”

Providers like InTime can provide the imagery of the fields that can be used to develop the prescriptions, noted Maas. “In some parts of the country, consultants can do the imagery, scout, and develop the prescriptions. So there are opportunities to get into this technology without having to buy a lot of equipment or go through a lot of computer manipulations.

“The important thing about variable-rate plant growth regulator application is that it actually seems to work in our area — even when you consider all the costs of collecting the information. The farmer in this example has decreased his management costs by about 10 percent and increased his yields by 10 percent.”